Within packaging technology, use is often made of packages of single use disposable type, and a very large group of these so called single use disposable packages is produced from a laminated, sheet or web shaped packaging material comprising a relatively thick bulk layer of e.g. paper or paperboard, and outer liquid tight coatings of plastic. In certain cases, in particular in conjunction with especially perishable and oxygen gas sensitive products, the packaging material also displays an aluminum foil in order to impart to the packages superior gas and light barrier properties.
Within food packaging, and especially within liquid food packaging prior art single use packages are most generally produced with the aid of modern packaging and filling machines of the type which both forms, fills and seals finished packages from the sheet- or web shaped packaging material. Such method may e.g. include a first step of reforming the packaging material web into a hollow tube. The tube is thereafter filled with the pertinent contents and is subsequently divided into closed, filled package units. The package units are separated from one another and finally given the desired geometric configuration and shape by a forming operation prior to discharge from the packaging and filling machine for further refinement process or transport and handling of the finished packages.
In order to facilitate the reforming of the packaging material into shaped packages the packaging material is provided with a suitable pattern of material weakening or crease lines defining the folding lines. In addition to facilitating folding the crease lines when folded also contribute to the mechanical strength and stability of the final packages; the packages may thus be stacked and handled without the risk of being deformed or otherwise destroyed under normal handling. Further to this the crease lines may also allow specific geometries and appearance of the packages.
Some different methods for providing crease lines have been proposed. For example, a method is known performing the step of introducing the packaging material in a nip between two driven rollers. One of the rollers is provided with a pattern of crease bars, while the other roller is provided with a corresponding pattern of recesses.
In the above-mentioned methods the packaging material is forced between rigid bars/recesses of pressing rollers. The packaging material will consequently be exposed to considerable stresses whereby the cellulose fiber structure of the packaging material may be partly disintegrated and thereby weakened.
The quality of the final package is of great importance, especially when it comes to liquid food packaging and aseptic packages. The packages are subject to very high requirements in order to ensure food safety, while at the same time the packages need to be robust and geometrically well-defined in order to improve storing and handling. The inventors have realized that the dimensional stability of the packages may be improved by using techniques configured to provide sharp edges and corners at the positions of the crease lines. With conventional creasing technology, a deeper imprint provides an improved crease and higher grip stiffness of a package produced with such folded creases. With deeper imprinted crease lines there will, however, be an increased risk of excessive disintegration of the bulk layer of the packaging material and even of cutting it or severely weakening it. In the case where the packaging material is laminated with a thin foil of aluminum acting as a barrier for oxygen, there is also an increased risk of crack formation in the aluminum foil, due to the deeper imprints causing air entrapments which make the aluminum foil weaker by being unsupported by adjacent layers.
Therefore this disclosure will present improved methods and systems for providing crease lines to a packaging material, which allows for obtaining dimensional stability of the final packages as well as improved or at least maintained quality and safety of the final packages.
In the forming process from web or tube to a box shaped package, so called flaps are formed, which are folded also along diagonal crease lines. Ideally, the corner part of the packaging material should have intersecting crease lines from longitudinal, transversal and diagonal directions. However, the prior art creasing methods cannot be designed for intersections with two or more crease lines in one single operation. The final folding into a prior art corner is therefore not entirely guided by crease lines but is also guided by tensions in the packaging material to compensate for missing parts or ends of creasing lines. As a consequence, a corner may exhibit unwanted defects, resulting in inferior package appearance. In some cases the tensions and deformations can cause cracks in a gas barrier layer, that can negatively affect the package integrity, including e.g. the gas barrier properties.
Hence, there is a need for an improved package overcoming the above-mentioned drawbacks of prior art packages.